The continuing evolution towards very large scale integrated circuit (VLSIC) and very high speed integrated circuit (VHSIC) technologies has caused exacting demands on the silicon epitaxial process in terms of reduced film thickness, minimized autodoping, low epitaxial temperatures, very low defect levels and good film uniformities. This trend applies continuous pressure on improving techniques in growing epitaxial layers on doped substrates. See for example, G. R. Srinivasan, "Silicon Epitaxy for High Performance Integrated Circuits," Solid State Technology, Nov. 1981, pp. 101-110.
One of the problems noted above involves the minimization of autodoping during growth of the epitaxial layers. Autodoping may be defined as undesired, uncontrolled doping of the epitaxial layer during growth where the dopant comes from an unintentional source, such as the reaction chamber, impurities in the epitaxial growth source, the susceptor used to support the wafers being processed, the doped substrate etching process or the doped substrate itself. It has been determined that out-diffusion from the doped substrate itself during epitaxial growth is the major contributor to the autodoping phenomenon. More specifically, the source of the dopant is the back side of the wafer as very little dopant is out-diffused through the front of the wafer once the epitaxial layer has started growing, under the relatively low temperature conditions (&lt;1000.degree. C.) employed in the current techniques. However, these conditions are still severe enough to cause gas-phase diffusion from the back side of the wafer.
FIG. 1 illustrates how epitaxial layer 10 may be contaminated by other dopants (donor or acceptors) diffusing from the back side 12 of doped substrate 14 along diffusion paths 16 during the growth of epitaxial layer 10. Autodoping is undesirable because it causes unpredictable changes in the impurity concentration profile of the epitaxial layer which can adversely affect the structure and performance of the devices to be placed on and in the epitaxial layer. It is typically preferred that the epitaxial layer be more lightly doped than the substrate.
Autodoping of epitaxial and other surface layers upon a doped substrate also occurs when the wafer is subjected to other processing typical of semiconductor device fabrication.
One prior art solution of the autodoping problem concerned altering the epitaxy growth process to minimize autodoping. In actual practice, this approach can only be partially successful in reducing autodoping during epitaxy growth and does not address autodoping which may occur during post-epitaxial layer processing. Another practiced solution has been back side capping of the substrate with a dopant diffusion barrier prior to epitaxy growth to prevent dopant diffusion out of the substrate back side. In this latter technique, the dopant diffusion barrier is deposited or grown only on the back side of the wafer, if possible, or formed on both sides of the substrate wafer and then removed only from the front side. In this discussion, the front side of the substrate will always be the side to receive the epitaxial layer.
D. C. Gupta, et al in "Silicon Epitaxial Layers with Abrupt Interface Impurity Profiles," Journal of the Electrochemical Society, Vol. 116, No. 11, November 1969, pp. 1561-1565 find that "back-sealing the substrates prior to deposition by using an insulating, etch-resistant film, e.g., silicon oxide, silicon nitride, etc.," significantly reduces the amount of impurity autodoping at the epitaxial layer/substrate interface. In this work, the substrate was doped with arsenic. The specific material used in back side sealing was not revealed.
However, B. A. Joyce et al in "Impurity Redistribution Processes in Epitaxial Silicon Layers," Journal of the Electrochemical Society, Vol. 112, No. 11, November 1965, pp. 1100-1106 find that although thermally grown oxide on the back of the substrate is considerably effective as an out-diffusion barrier for arsenic, phosphorus and antimony, some out-diffusion transfer occurs even if the substrate is completely oxidized. Further, the effectiveness of a thermal oxide film as a barrier to out-diffusion is very low in the case of gallium dopant, which is a Group III acceptor in contrast to phosphorus, arsenic and antimony which are Group V donors. Thus, it appears that although silicon oxide can serve as a dopant diffusion barrier to donors, its effectiveness varies with the particular donor and it is not completely effective for any acceptor.
Other barriers, such as silicon nitride proposed in passing by Gupta, et al noted above, have problems because they cannot be cleanly etched from the front side of the wafer. In other words, the dry etching techniques, such as plasma etching, necessary for removing silicon nitride from the front side of the wafer prior to epitaxial layer growth causes contamination and mechanical damage to the substrate front surface. Such physical defects in the surface of the substrate are well-known to propagate crystal lattice defects in the subsequent epitaxial layers, causing an inferior product.
Other layers are known to be deposited on the back side of the substrate or wafer, but these layers typically only serve as reverse bias electrode terminals. U.S. Pat. Nos. 4,485,553 and 4,468,857 to Christian et al describe a two-wafer method for manufacturing an integrated circuit device. Although multilayer protection is disclosed therein, including using a layer of silicon dioxide covered by a layer of silicon nitride, the protective layers are placed on a separate support wafer which is glued on top of a wafer which bears the semiconductor devices which have been already formed. Thus, the autodoping problem is not avoided by this technique.